Many time I've seen some material using mixing of two same nodes like this:
Is there any difference from using this?
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They are not equivalent. Mixing two different roughnesses produces a subtly different distribution of angles at which light can be scattered:
Note the shaper highlights on the right hand example (mixed roughness).
To visualize what is going on, take this (very approximate) example:
The length of the arrows represents the amount of light that gets scattered in that direction.
Late to the party but the example with suzanne is not very clear.
Let me over-exaggerate and demonstrate - these are the 2 shaders that will participate - glossy sphere with a perfect point light in front (0.0 size):
Now let's mix the roughnesses into 1 shader ((0.01+0.91)/2 = 0.465) and also mix the shaders to see the difference:
The mixed shaders doesn't produce a single "material" (no physically correct material has this roughness distribution), but they produce (simulate) two materials - a material with a glossy clear coat. That's what it is used for.